Energy is the material basis of human activities. With the continuous development and progress of society, the demand for energy is increasing. The traditional fossil energy, belonging to non-renewable energy, has been difficult to meet the demands of the social development. Therefore, in recent years, new energy and renewable energy have been widely researched and utilized in countries all over the world. Among others, the solar power generation technology has attracted much attention due to its advantages of capability of converting sunlight into electric power, easy operation, environmental protection and no pollution, and high energy utilization. Solar power generation is a process of power generation in which large-area P-N junction diodes are used to produce photon-generated carries under the radiation of sunlight.
In the prior art, for dominant and highly commercial crystalline silicon solar cells, the emitter region and electrodes in the emitter region are all on the front side (light-facing side) of the cell. That is, the main gate line and the auxiliary gate line are both on the front side of the cell. Since solar-grade silicon material has a short diffusion distance, placing the emitter region on the front side is helpful for improving the collection efficiency of carriers. However, since the gate lines on the front side of the cell shield part of sunlight (about 8%), the effective light-receiving area of the solar cell is reduced and part of current is thus lost. In addition, when cells are connected in series to each other, a tin-plated copper band is to be welded from the front side of one cell to the back side of another cell. If a thick tin-plated copper band is used, it may be possible to crack the cell because of its excessive hardness. However, if a thin but wide tin-plated copper band is used, much sunlight may be shielded. Therefore, the loss resulted from the serial connection of resistors and the optical loss will be caused regardless of the type of the tin-plated copper band used. Also, the use of the tin-plated copper band is disadvantageous to the thinning of the cell. To solve the above technical problems, by moving electrodes on the front side to the back side of the cell, those skilled in the art have developed a main-gate-free back-contact solar cell. A back-contact solar cell is a solar cell in which electrodes in the emitter region and electrodes in the base region of the cell are all on the back side of the cell. Such a back-contact cell has many advantages. Firstly, high efficiency: since the loss resulted from the light shielding by the gate line electrodes on the front side is completely eliminated, the efficiency of the cell is improved. Secondly, the thinning of the cell can be realized. Since metallic connection devices used for serial connection are all on the back side of the cell and there is no connection from the front side to the back side, a thinner silicon wafer can be used and the cost can be thus reduced. Thirdly, it is more beautiful and the color of the front side of the cell is uniform. The aesthetic requirements of the customers are satisfied.
The back-contact solar cells comprise many structures, for example, MWT, EWT and IBC. How to connect back-contact solar cells in series to form a solar cell assembly in high efficiency and at low cost is the key to realize highly commercial production of the back-contact solar cells. A conventional method for preparing an MWT assembly is to prepare conductive backing composite material, apply conductive adhesive on the conductive backing material, punch packaging material at a corresponding position so that the conductive adhesive penetrates through the packaging material, accurately place the back-contact solar cell on the packaging material so that conductive points on the conductive backing material come into contact with electrodes on the back-contact solar cell by the conductive adhesive, and then pave an upper layer of EVA and glass on the cell, and finally overturn the whole well-stacked module and put it into a laminating press for lamination. This process has the following several shortcomings. Firstly, the used conductive backing composite material is obtained by compositing conductive metal foil, usually copper foil, to the backing material, and it is required to perform laser etching or chemical corrosion on the copper foil. Since laser etching is slow in etching complex patterns although feasible for simple patterns, the production efficiency is low. And, with regard to chemical corrosion, it is required to prepare masks with complex shape and corrosion resistance property in advance, and the chemical corrosion also cause environmental pollution and corrosion to polymeric base material by the corrosive liquid. The conductive backing material prepared in this way is complex in preparation process and extremely high in cost. Secondly, it is required to punch the packaging material behind the solar cell so that the conductive adhesive penetrates through the packaging material. Since the packaging material is usually viscoelastic material, it is difficult to perform precise punching. Thirdly, a precise dispensing apparatus is required to coat the conductive adhesive to the corresponding position of the backing material. It is feasible for MWT cells with less back contacts. In contrast, for back-contact cells with a large amount of back contacts each having a small area, it is impossible.
In the IBC technology, since the P-N junctions are placed on the back side of the cell, without any shield on the front side, and meanwhile, the electron collection distance is reduced, the efficiency of the cell can be significantly improved. For IBC cells, shallow diffusion, light doping, SiO2 passivation layers, etc., are used on the front side to reduce the compositing loss, while on the back side of the cell, the diffusion regions are limited within a small region. Those diffusion regions are arranged in a lattice on the back side of the cell. Metallic contacts in the diffusion regions are limited within a very small area, appearing as a great number of small contacts. With regard to IBC cells, since the area of heavy diffusion regions on the back side of the cell is reduced, the saturation dark current in the doped region can be greatly reduced, and the open-circuit voltage and the conversion efficiency can be improved. Meanwhile, collecting current by a great number of small contacts reduces the transfer distance of the current on the back side and greatly decreases the internal series resistance of the assembly.
IBC back-contact cells have attracted much attention since they provide for high efficiency which is difficult to realize for the conventional solar cells, and have become a research hotspot of a new generation of solar cell technology. However, in the prior art, the P-N junctions in the IBC solar cell modules are positioned adjacent or close to each other and all on the back side of the cell. Accordingly, it is difficult to connect the IBC cell modules in series to form an assembly. In order to solve the above problem, there have been many improvements to the main-gate-free back-contact IBC solar cells in the prior art. In Sunpower Corp., the adjacent P or N emitters are connected by small gate lines obtained from silver paste by screen printing so that the current is guided to the edge of the cell; and big solder joints are printed on the edge of the cell, and then welded and connected in series by a connection band. At present, in the solar energy field, busbars for the current are usually formed by screen printing, for example, the newly applied patents 201310260260.8, 201310606634.7, 201410038687.8, 201410115631.8.
Patent WO2011143341A2 disclosed a main-gate-free back-contact solar cell, comprising a substrate; several adjacent P-type doping layers and N-type doping layers are located on the back side of the substrate; the P-type doping layers and the N-type doping layers are stacked with a metallic contact layer, and a passivation layer is provided between the P-type doping layers and N-type doping layers and the metallic contact layer; and a great number of nano-level connection holes are formed on the passivation layer, and the nano-level connection holes connect the P-type doping layers and N-type doping layers to the metallic contact layer. However, in this invention, connecting the metallic contact layer by nano-level holes will increase the resistance, the preparation process is complex, and high requirements are proposed to the preparation apparatus. In this invention, it is unable to integrate a number of solar cells and the electrical connection layer to one module. The integration of cells into solar cell modules is convenient to assemble the solar cell modules into an assembly, and also convenient to adjust the series/parallel connection between the modules. In this way, it can be convenient to adjust the series/parallel connection between cells in the solar cell modules, and reduce the connection resistance of the assembly.
Patent US20110041908 A1 disclosed a back-contact solar cell having, on its back side, elongated emitter regions and base regions which are interleaved, and a method for producing the same. The back-contact solar cell has a semiconductor substrate; elongated base regions and elongated emitter regions are provided on the surface of the back side of the semiconductor substrate, the base regions having a base semiconductor type, and the emitter regions having an emitter semiconductor type opposite to the base semiconductor type; the elongated emitter regions have elongated emitter electrodes for electrically contacting the emitter regions, and the elongated base regions have elongated base electrodes for electrically contacting the base regions, wherein the elongated emitter regions have smaller structural widths than the elongated emitter electrodes, and wherein the elongated base regions have smaller structural widths than the elongated base electrodes. The elongated conducting members used in this invention allow the solar cell to have excellent current collection performance. However, it is necessary to provide a large number of conducting members to effectively collect the current. Therefore, the manufacturing cost is increased, and the process steps are complex.
Patent EP2709162A1 disclosed a solar cell, used in a main-gate-free back-contact solar cell, and disclosed electrode contact units which are spaced apart from each other and arranged alternately. The electrode contact units are connected by longitudinal connectors to form an “I”-shaped electrode structure. However, this structure forms two connections on the cells. The first connection is to connect the cells to the electrode contact units, and the second connection is to connect the electrode contact units by connectors. The two connections result in complex process and too many electrode contacts. As a result, “disconnection” or “misconnection” may be caused. This is disadvantageous to the overall performance of the main-gate-free back-contact solar cell.
At present, in the inventions in the art, small gate lines are used for current collection. This is feasible for 5-inch cells. However, for 6-inch or bigger silicon wafers that are popular in the prior art, problems such as the rise of the series resistance and the reduction of the filling factor may occur. Consequently, the power of the manufactured assembly is significantly decreased. For IBC cells in the prior art, wider gate lines made of silver paste can be formed between the adjacent P or N emitters by screen printing to reduce the series resistance. However, the increase of the silver amount causes the sharp increase of the cost, and meanwhile, wide gate lines result in deteriorated insulating effect between P and N emitters and easy current leakage.